Infection with Vibrio fetus venerealis produces a naturally venereal disease in cattle which in essential features closely resembles gonorrheal infection. A humoral immune response, of which IgA is an important component, occurs in convalescent animals. In addition, a high degree of resistance toward vibriosis, correlated with the transudation of IgG antibodies, can be produced by parenteral immunization with bacterin in Freund's complete adjuvant (CFA). The following experiments are proposed with V. fetus: 1. The effectiveness of parenteral immunization in terminating the carrier state and in protecting animals infected during different stages of the reproductive cycle will be determined; immunizing agents composed of purified antigens will be tested and the incorporation of vaccines into vehicles other than CFA will e evaluated. 2. The surface antigens (capsular and flagellar) of the organism will be characterized and extracellular components which may function as virulence mechanisms sought. 3. The protective function of humoral and cellular mechanisms of immunity in infections with this organism will be assessed.